The gate insulation film of a MIS (metal/insulator/silicon) transistor is required to have various high-performance electric properties and high reliability characteristics, such as low leakage current characteristics, low interface state density, high breakdown voltage, high resistance against hot carriers, and uniform threshold voltage characteristics.
The thermal oxidation technology using oxygen molecules or water molecules at approximately 800° C. or more has been used conventionally as the formation technology of the gate insulation film that satisfies the above requirements.
A thermal oxidation process has been conducted conventionally after conducting a cleaning process of removing surface contaminants, such as organic materials, metals, and particles, as a preprocessing process. In such a conventional cleaning process, cleaning using a diluted hydrofluoric acid or hydrogenated water, for example, is performed at last, for terminating the dangling bonds existing on the silicon surface by hydrogen. Thereby, formation of a native oxide film on the silicon surface is suppressed, and the silicon substrate thus having a clean surface is forwarded to the following thermal oxidation process. In the thermal oxidation process, the terminated hydrogen at the surface undergoes decoupling during the process of raising the temperature of the silicon substrate in an inert gas atmosphere of argon (Ar), for example at a temperature equal to or more than 600° C., approximately. Then, oxidization of the silicon surface is conducted at approximately 800° C. or more in an atmosphere to which oxygen molecules or water molecules are introduced.
Conventionally, in a case where a silicon oxide film is formed on the silicon surface by using such a thermal oxidization technique, satisfactory oxide film/silicon interface characteristics, high breakdown voltage of the oxide film, leakage current characteristics, and the like, are achieved only in the case where a silicon surface having the (100) orientation is used. Further, remarkable degradation of leak current occurs in the case where the thickness of the silicon oxide film formed by the conventional thermal oxidation process is reduced to approximately 2 nm or less. Thus, it has been difficult to realize a high-performance miniaturized transistor that requires decrease of the gate insulation film thickness.
Further, in a crystal silicon having a surface orientation other than the (100) orientation or a polycrystalline silicon generally having a primarily (111)-oriented surface on an insulation film, interface state density at the oxide film/silicon interface is remarkably high as compared with the silicon oxide film formed on the (100)-oriented silicon even when the silicon oxide film is formed by using the thermal oxidation technology. Thus, a silicon oxide film having a reduced film thickness possesses poor electric properties in terms of breakdown characteristics, leakage current characteristics, and the like. Hence, there has been a need of increasing the film thickness of the silicon oxide film when using such a silicon oxide film.
Meanwhile, the use of large-diameter silicon wafer substrate or large-area glass substrate is increasing these days for improving the efficiency of semiconductor device production. In order to form transistors on the entire surface of such a large-size substrate with uniform characteristics and with high throughput, an insulation film forming process conducted at a low temperature so as to decrease the magnitude of the temperature change in heating or cooling and, further, having small temperature dependence is required. In the conventional thermal oxidation process, there has been a large fluctuation of oxidation reaction rate with respect to temperature fluctuation, and it has been difficult to produce semiconductor devices with high throughput while using a large-area substrate.
In order to solve these problems associated with the conventional thermal oxidation technology, multitudes of low-temperature film formation processes have been attempted. Among others, the technology disclosed in Japanese Laid-Open Patent Publication No. 11-279773 or the technology disclosed in Technical Digest of International Electron Devices Meeting, 1999, pp. 249–252, or in 2000 Symposium on VLSI Technology Digest of Technical Papers, pp. 76–177, describes a process in which an inert gas is introduced into plasma together with gaseous oxygen molecules, thereby effectively causing the inert gas having a large metastable level to conduct the atomization of the oxygen molecules. Hence, relatively good electronic properties are achieved.
In these technologies, a microwave is irradiated to the mixed gas formed of krypton (Kr) that is an inert gas and an oxygen (O2) gas, the mixed plasma of Kr and O2 is generated, and a large amount of atomic state oxygen O* are formed. Then, the oxidation of silicon is conducted at a temperature of about 400° C., and low leakage current characteristics, low interface state density, and high breakdown voltage comparable to those of the conventional thermal oxidation are achieved. Further, according to this oxidation technology, a high-quality oxide film is obtained also on the silicon surface having a surface orientation other than the (100) surface.
However, in such a conventional silicon oxide film formation technology using the microwave-excited plasma, in spite of the fact that the oxidation is conducted by using atomic state oxygen O*, only a silicon oxide film having electric properties comparable to those obtained by the conventional thermal oxidation process that uses oxygen molecules or water molecules is obtained. Particularly, it has been impossible to obtain the good low leakage current characteristics in the silicon oxide film having a thickness of approximately 2 nm or less on the silicon substrate surface. Thus, it has been difficult to realize high-performance, miniaturized transistors that require further decrease of the gate insulation film thickness, similarly to the case of conventional thermal oxide film formation technology.
Further, there has been a problem that degradation of conductance caused by hot carrier injection into the oxide film of a transistor, or degradation of electric properties with time such as increase of leakage current, in a device that causes tunneling of electrons through the silicon oxide film as in the case of a flash memory, occur more noticeably than in the case where the silicon oxide film is formed by the conventional thermal processes.